Process for making metal articles



Oct. 10, 1939. J. M. KELLY PROCESS FOR MAKING METAL ARTICLES Filed July 51, 19:7

INVENTOR fi zlq. 2.

WITNESSES:

Patented Get. 10, 1939 PROCESS FOR MAKING METAL ARTICLES James M. Kelly, Trailord, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 31, 1937, Serial No. 156,766

1 Claim.

This invention relates to the making of metal articles, and particularly to the process of making electrical contact members.

In manufacturing electrical contact members of refractory metals, such as tungsten, molybdenum or tantalum bonded with the highly conductive metals copper and silver, the metals are usually powdered and the refractory metal powders are compacted under pressures of the order of up to 25 tons into a slug of suitable shape and size. In order to impregnate the slug of compacted refractory metal powders, the slug is then placed in a suitable vessel and surrounded by sheets or bodies of the highly conductive metal which upon the application of sufficient heat melts and flows into the voids in the compacted slug.

It is not always possible to secure the characteristics desired in) the contact member by making them by the known processes, since it is difficult to control the degree of impregnation of the highly conductive metals in the refractory metal slug and there are definite limits as to the amount of the highly conductive metal it is possible to introduce into the slug of refractory metal. The compacting of the refractory metal powders under the large pressures in accordance with prior practice has also greatly added to the cost of manufacturing the elements necessitating separate and careful handling of the compacted refractory slug in placing it in the impregnating vessel.

An object of this invention is to provide for making articles of predetermined shape from metal powders.

Another object of this invention is to provide an improved novel and commercially desirable process of producing an article compounded from metallic powders.

A further object of this invention is to provide for making articles of manufacture compounded from metallic powders and simultaneously shaped, compacted and bonded in a mold of suitable refractory material which is protected against oxidation at high temperatures.

A more specific object of this invention is to provide a process for making dense articles of predetermined shape of metal powders in predetermined proportions for use in different applications and particularly as electrical contacts or electrodes for use in Welding operations.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing, in which:

Figure 1 is an elevational view in cross section of apparatus for practicing the process of this invention, and

Fig. 2 is an enlarged view in cross section of apparatus illustrating the making of a compacted electrode in accordance with the teachings of this invention. v

In practicing this invention, the metals to be compounded into the articles of predetermined shape are powdered and screened to a desirable size, after which the metal powders in specified proportions may be thoroughly. and intimately mixed in any suitable manner. Milling the metal powders in specified proportions in a ball mill of appropriate size produces a thorough and intimately mixed powder mixture suitable for use as hereinafter described.

Referring to the drawing and Fig. 1 in particular, a representative apparatus employed in compounding the articles of predetermined shape from the metal powders is illustrated. The apparatus comprises a mold ID of suitable refractory material such as graphite which is capable of withstanding the efiects of large changes in temperature. The mold II] is provided with a cavity of the shape of the desired articlefor receiving a charge l2 of the loose intimately mixed metal powders in the specified proportions. In order to provide for the protection of the metal powders in the mold and to prevent oxidation of the graphite mold at the high temperatures of the heat treatment, the mold is disposed in a substantially closed vessel or heating chamber I4 having a removable cover I6 to which a nonoxidizing protective gas such as hydrogen, nitrogen or cracked ammonia may be suppiled as through an inlet I8. The vessel or chamber I4 may be heated in any suitable manner by any of the common expedients known to the art.

As illustrated, the mold for containing the intimately mixed metal powders is provided with an orifice in its top for admitting the powders to the cavity. Since the metal powders are loosely placed in the cavity of the mold, in order to produce a dense article it is desirable to so apply sufiicient pressure to the powders that when heated to a temperature above the melting point of the lower melting constituent of the powders they are compacted into a dense article of predetermined shape. As illustrated in Fig. l, a plunger or rod 20 of suitable refractory material, such as graphite, and having a contacting surface of a size to fit in the orifice of the mold and seat on the metal powders may be employed for loading the powders to compact them. The plunger 20 may extend through an opening in the cover of the vessel, as shown, for facilitating the application of a force, such as a weight 22, to the plunger while subjecting the powders to a predetermined temperature.

In practicing this process, the metal powders are intimately mixed in specified proportions in a ball mill, as referred to hereinbefore and loosely placed into the cavity of the mold It! with the contacting surface of the plunger 20 seating on the powders. The protective gas may be admitted to the chamber I I when the cover is secured to the chamber after which the chamber is heated by any suitable means, not shown, to a temperature above the melting point of the lower melt ing constituent of the powders. At the same time a weight for applying a suitable force to the powders in the mold may be applied to the graphite plunger, so that as the temperature within the chamber is raised above the melting point of the lower melting metal, the rod or plunger 20 will be forced downwardly maintaining its seat on the contacting surface of the powders and compressing the powders into a dense object with the lower melting metal flowing about the particles of the other metals comprising the article.

Through experiments, it has been found that a weight of about 30 lbs. is sufflcient when applied to the end of the plunger to so compact the powders into a slug having a diameter of .511 inch and a length of .54 inch when subjected to the heat, as described, for a period of time of about eight to twenty minutes. The slug has a density of about 85% of the true density with only about of voids. The load applied to the powders in the mold varies with the size of the slug or article which it is desired to produce and the design of the mold employed. The load or pressure necessary for producing an article of a given size, shape and desired density may be easily determined through experiments.

Although as a general rule, the metals do not react with the graphite mold, it may sometimes be desirable to provide a protective coating to prevent any possible reaction of the metals with the refractory material of the mold. This may be accomplished by applying a refractory wash over the mold surface of the cavity. A wash comprising alundum, zirconia, or calcium carbonate carried in either water or alcohol is satisfactory as a protective coating of the mold surface of the cavity. When applied to the mold surface the carrier of the refractory wash quickly evaporates leaving a coating of alundum, zirconia or calcium carbonate on the mold surface.

This invention may be employed in making artlcles of different metals for different applications. Satisfactory electrical contact elements may be produced by this process from the refractory metals, tungsten, molybdenum or tantalum and the highly conductive metals copper and silver in any specified proportion depending upon the characteristics desired in the element. The proportions of the refractory metals employed depends upon the strength, machinability, heat and electrical conductivity desired.

Excellent results have been secured by employing this process to produce articles comprising from 10 to 90% of the refractory metal with from 90 to 10% of the highly conductive metal. Examination of the articles produced from the refractory metals and the highly conductive metals revals a uniform dense structure. There is, however, no evidence of alloying when the intimate mixtures ofthe refractory metal and highly conductive metals are subjected to this process. This is probably because there is no appreciable solid solubility of one element in the other.

Referring to Fig. 2 of the drawing, the mak ing of a composite electrode compacted from different metallic powders is illustrated. In this embodiment of the invention, the mold H] has a cavity of the shape of the desired electrode.

In making the electrode, it is desired to have a contact tip of hard material with a shank of softer material having high conductivity. In making the integral composite electrode, a quantity 24 of screened refractory metal powder having a. minor proportion of hi'ghly conductive metal powder intimately mixed therewith is placed in the bottom of the cavity of the mold. In order to produce a shank on the electrode having a high conductivity, metallic powders 28 of the highly conductive metal with or without a minor proportion of refractory metal powder intimately mixed therewith, as described, may be placed on top of the quantity of refractory powders in the bottom or tip of the cavity. The mass of loose powders in the mold may then be subjected to a temperature above the melting point of the highly conductive metal for a period of time of about ten minutes and subjected to pressure applied through the plunger 20, as described hereinbefore, to effect a compacting of the powders in the mold.

Where a minor proportion of the highly conductive metals is employed in the tip of the electrode, it is found that the lower melting and highly conductive metal melts and flows around the particles of the refractory metal powders in the tip, securely binding them while a small amount of the lower melting metal from the shank of the electrode flows downwardly bonding the refractory metal tip to the shank. The pressure applied through the plunger functions during the melting of the low melting metal to compact the powders into an integral dense unit. Electrode tips having a diameter of .75 inch and a length of about 2.5 inches in which 80% tungsten powders were employed in the tip, as described, compounded under pressure of up to 50 pounds, have been found to have a hardness of 93 Rockwell B and a density of about 90% of the true density.

Although described with reference to the making of articles from refractory metal pow ders and highly conductive metal powders, the process of this invention may be employed in making other and various articles of manufacture from different metallic powders. For example, dense brass castings having a very fine degree of sharpness may be produced from zinc and copper powders in the proper proportions by practicing the process of this invention. This invention is, therefore, not to be limited except insofar as is necessitated by the prior art and the scope of the appended claim.

I claim as my invention:

A process for making welding electrodes comprising intimately mixing a major proportion of a refractory metal powder with a minor proportion of a highly conductive metal powder, placing the mixture in a mold of refractory material, intimately mixing a major proportion of a highly conductive metal powder and a minor proportion of a refractory metal powder, placing said second mixture on top of said first mixture in intimate relation thereto in the mold of refractory material, and heating the metal powders in the mold while under pressure to produce a dense, composite electrode having a contact tip of refractory metal and a shank of highly conductive metal, the heating being in a protective atmosphere at a temperature above the melting point of the highly conductive metal to cause the highly conductive metal to melt and bond the powders into an integral unit.

JAMES M. KELLY. 

